Generally, tobacco products are produced from a material obtained by blending various types of leaf tobacco. Blending is a process generally carried out not only for tobacco products but also for various food products such as products of coffee, tea, rice and wheat flour. Because leaf tobacco is an agricultural product, amounts of components in the leaf tobacco vary every year depending on weather conditions. However, blending a various types of leaf tobacco as appropriate makes it possible to reproduce a material having a target quality. This allows providing products having a stable quality. Further, if development of leaf tobacco can provide leaf tobacco having different components in terms of quantity and quality from those of conventional leaf tobacco, a range of taste and flavor created by blending will be able to extend. This makes it possible to further develop various new products. Presently, a diversity of leaf tobacco is created by combinations of varieties, cultivation methods, curing methods, storage/fermentation methods, production regions, stalk positions and the like. For further extending possibilities of such blending techniques, it is desired to develop new leaf tobacco having different components, for example, components relevant to flavor and smoking taste, in terms of quantity and quality from those of conventional leaf tobacco.
Examples of components relevant to flavor and smoking taste in leaf tobacco are sugars, amino acids, organic acids, phenolic compounds, terpenoids, and alkaloids (nicotine).
Among the above components, nicotine is one of main components of leaf tobacco. Leffingwell reports that respective nicotine contents of Nicotiana tabacum and Nicotiana rustica are in a range of 0.2% to 8% (Non-Patent Literature 1). The nicotine contents in leaf tobacco vary in a wide range due to great influences from not only genetic factors that each variety has but also environmental factors such as meteorological factors and edaphic factors, and cultivation factors such as fertilization methods, topping methods, and harvesting methods.
Among such factors, our understanding on genetic factors has greatly progressed in recent years, as a result of development of molecular biology and genetic recombination techniques. Such progress has lead to identification of many genes that influence nicotine contents of Nicotiana plants.
For example, Sato et al. report transgenic plants (Nicotiana sylvestris) whose leaf nicotine contents are decreased by suppression of expression of putrescine methyl transferase (PMT) gene or increased by overexpression of the PMT gene (Non-Patent Literature 2). Xie et al. report transgenic plants whose leaf nicotine contents are decreased by suppression of expression of quinolate phosphoribosyl transferase (QPT) gene (Non-Patent Literature 3). Hashimoto et al. report transformed tobacco hairy roots (variety K326) whose nicotine contents are increased by overexpression of one or both of A622 and NBB1 genes and transformed tobacco plants (variety K326) whose leaf nicotine contents are increased by overexpression of one or both of PMT and QPT genes (Patent Literature 1). Further, Hashimoto et al. report transformed tobacco hairy roots (variety SR-1) whose nicotine contents are decreased by suppressing expression of N-methyl putrescine oxidase (MPO) gene and transformed tobacco cells (BY-2) whose nicotine contents are increased by overexpression of the MPO gene (Patent Literature 2). Furthermore, Hashimoto et al. report transformed tobacco cells (BY-2) and transformed tobacco hairy roots (variety Petit Havana SR1) nicotine contents of which transformed tobacco cells and transformed hairy roots are decreased by suppressing expression of A622 gene or NBB1 gene and also reports transformed tobacco plants (variety Petit Havana SR1) whose leaf nicotine contents are decreased by suppressing expression of the NBB1 gene (Patent Literature 3). Chintapakorn and Hamill report transformed hairy roots (variety NC-95) whose nicotine content is decreased by suppressing expression of arginine decarboxylase (ADC) gene (Non-Patent Literature 4). Hakkinen et al. report transformed tobacco cells (BY-2) whose nicotine contents are increased by overexpression of MAP2, MC126 or MT401 gene and transformed tobacco hairy roots (variety BY-2) whose nicotine contents are increased by overexpression of C127 gene (Non-Patent Literature 5). Shoji et al. report transformed tobacco plants whose leaf nicotine contents are decreased by suppressing expression of COI1 gene (Non-Patent Literature 6). Wang et al. report transformed plants whose leaf nicotine contents are decreased by concurrently suppressing expression of JAR4 and JAR6 genes in Nicotiana attenuata (Non-Patent Literature 7). Bailey et al. report transformed tobacco plants whose nicotine contents are increased by overexpression of VHb gene (Patent Literature 4). Inze et al. report transformed tobacco cells (BY-2) whose nicotine contents are increased by overexpression of MAP3 gene (Patent Literature 5). Page and Todd report transformed plants (Nicotiana benthamiana) whose nicotine contents are decreased by suppressing expression of NbTF1, NbTF4, or NbTF5 gene encoding a transcription factor and transformed plants (Nicotiana benthamiana) whose nicotine contents are increased by overexpression of the NbTF1, NbTF4, or NbTF5 (Patent Literature 6).
Regarding biosynthesis and an accumulation mechanism of nicotine in Nicotiana plants, a lot of physiological studies have been made for ages. As a result, involvement of plant hormones such as auxin, jasmonic acid, salicylic acid, and ethylene has become evident. For example, Solt (Non-Patent Literature 8), Yasumatsu (Non-Patent Literature 9), Mizusaki et al. (Non-Patent Literature 10), and Takahashi and Yamada (Non-Patent Literature 11) report that auxin negatively regulates the biosynthesis or accumulation of nicotine. Further, Baldwin et al. (Non-Patent Literature 12) report that salicylic acid negatively regulates accumulation of nicotine. Furthermore, Baldwin et al. (Non-Patent Literature 13), Imanishi et al. (Non-Patent Literature 14), and Goossens et al. (Non-Patent Literature 15) report that jasmonic acid positively regulates biosynthesis and accumulation of nicotine. Shoji et al. (Non-Patent Literature 16) and Kahl et al. (Non-Patent Literature 17) report that ethylene negatively regulates biosynthesis or accumulation of nicotine.
Such involvement of various plant hormones indicates that biosynthesis and accumulation of nicotine is regulated by complex networks including a plurality of signaling systems and a plurality of transcriptional regulation systems. Such networks are reported by Kazan and Manners (Non-Patent Literature 18), for example. Among the above-described genes that influence nicotine contents of Nicotiana plants, for example, COI1, JAR4, JAR6, MAP3, NbTF1, NbTF4, and NbTF5 genes are considered to be not genes encoding nicotine biosynthetic enzymes but genes involved in signaling or transcriptional regulation.
Plant hormones act on various aspects of various life processes. For example, plant hormones act on growth and regulation of morphology of plants, regulation of a secondary metabolic system, and regulation of response to biological/non-biological stresses. Accordingly, genes affecting leaf nicotine contents via signaling or transcriptional regulation of the plant hormones may vary not only nicotine but also other components in plants. The genes having such functions are important in increasing a diversity of leaf tobacco as described above. Accordingly, studies have been continued for identification of not only known genes but also new genes.